GOptionContext: add some notes about encodings
GOptionContext: add some notes about encodings

Add a note to the overview documentation for GOptionContext about why
you need to be careful about argv encoding on UNIX and about why you
should avoid argv entirely on Windows.  Mention some possible
alternative approaches, including a code example.

https://bugzilla.gnome.org/show_bug.cgi?id=722025

MathieuDuponchelle/glib,johne53/MB3Glib,krichter722/glib,gale320/glib,cention-sany/glib,johne53/MB3Glib,mzabaluev/glib,MathieuDuponchelle/glib,lukasz-skalski/glib,gale320/glib,johne53/MB3Glib,tamaskenez/glib,ieei/glib,ieei/glib,cention-sany/glib,ieei/glib,tamaskenez/glib,gale320/glib,tamaskenez/glib,tchakabam/glib,MathieuDuponchelle/glib,mzabaluev/glib,MathieuDuponchelle/glib,endlessm/glib,mzabaluev/glib,ieei/glib,cention-sany/glib,gale320/glib,gale320/glib,mzabaluev/glib,lukasz-skalski/glib,Distrotech/glib,krichter722/glib,mzabaluev/glib,tamaskenez/glib,krichter722/glib,Distrotech/glib,cention-sany/glib,tamaskenez/glib,endlessm/glib,tchakabam/glib,Distrotech/glib,johne53/MB3Glib,MathieuDuponchelle/glib,johne53/MB3Glib,tchakabam/glib,endlessm/glib,johne53/MB3Glib,cention-sany/glib,Distrotech/glib,endlessm/glib,lukasz-skalski/glib,ieei/glib,tchakabam/glib,endlessm/glib,krichter722/glib,lukasz-skalski/glib,lukasz-skalski/glib,krichter722/glib,tchakabam/glib,Distrotech/glib